Wireless communication systems are being widely deployed in order to provide various kinds of communication services, such as voice or data. In general, a wireless communication system is a multiple access system which can support communication with multiple users by sharing available radio resources (bandwidth, transmission power, etc.). The multiple access systems may include, for example, a Time Division Multiple Access (TDMA) system, a Code Division Multiple Access (CDMA) system, a Frequency Division Multiple Access (FDMA) system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, and a Single Carrier Frequency Division Multiple Access (SC-FDMA) system.
One or more cells are deployed in one base station included in the wireless communication system. A number of user equipments can be placed in one cell. In order for such a user equipment (UE) to access a network, the UE experiences a random access procedure. The random access procedure is performed through a Random Access Channel (RACH) (i.e., an uplink transmission channel).
The random access procedure can be classified into a contention-based random access procedure and a non-contention-based random access procedure. The random access procedure is schematically described below. When a UE sends a random access preamble, a base station sends a random access response to the UE. The UE sends a scheduled message to a network. The purpose of the UE to perform a random access procedure for a network can include initial access, handover, a scheduling request, timing alignment, and so on.
A conventional wireless communication system, in general, uses one bandwidth in order to send data. For example, the 2nd generation wireless communication system uses the bandwidth of 200 kHz to 1.25 MHz, and the 3rd wireless communication system uses the bandwidth of 5 MHz to 10 MHz. TO support an increasing capacity of data transmitted and received, a recent 3GPP LTE or 802.16m continues to extend its bandwidth up to 20 MHz or more. To extend the bandwidth in order to increase the capacity of data transmitted and received may be indispensable. However, to support a great bandwidth even when a required quality of service is low may result in high power consumption.
Therefore, there is emerging a multi-carrier system which is capable of defining a carrier having one bandwidth and a central frequency and transmitting and/or receiving data using a broadband through a plurality of carriers. Both a narrow band and a broadband are supported using one or more carriers. For example, if a single carrier corresponds to the bandwidth of 20 MHz, the bandwidth of 40 MHz can be supported using two carriers.
In such a multi-carrier system, in the case in which the random access procedure is performed, how the random access procedure will be performed according to which method becomes problematic when a number of carriers exist. For example, it may be problematic how the UE will send a random access preamble through which uplink carrier and will receive a random access response through which downlink carrier.